Correction of the gain and phase response of a power amplifier used to amplify an RF signal transmitted by an RF transmitter (or RF transceiver) can greatly improve the quality of the RF signal, and hence the performance of the RF transmitter. Under steady-state operating conditions, an amplifier and its associated circuitry generally perform in an expected, well behaved, and steady manner. Most PAs are designed and tuned so that, if a specific gain bias current (or voltage) is applied to it, it amplifies a signal with a predetermined and steady gain and phase response. This steady gain and phase response holds in general if the PA is operating under steady-state operating conditions, but does not necessarily apply under changing operating conditions.
Changing conditions can lead to changing electromagnetic, electrical, and other physical characteristics of the PA, its constituent components, or its associated circuitry. Changes in these physical characteristics can lead to an exhibited gain or phase response of the PA which does not correspond to that which normally accompanies the specific gain bias being applied to the PA. If a physical characteristic fluctuates, and if the gain or phase of the PA is affected by that physical characteristic, so will the exhibited gain or phase response of the PA. As such, changes in the operating conditions of the PA can lead to unwanted changes in the gain or phase response of the PA. Transitioning from an idle state to a full-on state can create a host of various, and possibly interdependent or causally related physical changes which lead to unwanted changes in the gain or phase response of the PA during the transition period.
It should be noted that one consideration for the performance of a PA used in an RF transmitter is what is known as the error vector magnitude (EVM) of the RF signals transmitted. The EVM is a measure of the error in the transmitted signal, characterized by the magnitude of error in the transmitted signal symbol's constellation points versus the constellation point locations of the input signal symbol. The EVM performance of a power amplifier can be measured in terms of the contribution to the EVM of the transmitter created by the amplification applied by the amplifier. All PA's contribute to the EVM to some degree as no PA is an ideal amplifier. Keeping the EVM caused by a PA as small as possible is an important goal in the design and manufacture of PAs.
Although various fluctuations in physical characteristics caused by changing operating conditions can affect the gain or phase response of the PA, the temperature of the PA has a profound affect on its response.
The EVM of a PA can increase under pulsed conditions. This increase in EVM is also known as dynamic EVM. This dynamic EVM primarily originates from temporal variations in the gain or phase response of the amplifier when it is experiencing transience, particularly during transitions from idle to steady-state operating conditions.
Due to its profound affect on the gain and phase response of the PA, one main contributor to unwanted changes in a PA's response and dynamic EVM is thermal changes of the PA caused by dynamic heating effects. One particular situation for which this can occur, is when the PA receives an RF signal data burst, after it has remained idle long enough that its temperature has fallen below its steady-state operating temperature. In one standard RF transmitter application, a Tx enable is received 500 ns to 1 μs before the RF signal data to be transmitted is received. Dynamic EVM manifesting as an increase in the EVM occurs primarily at the beginning of the pulse sequence, when the PA is coolest and hence at a temperature farthest from its steady-state operating temperature.
In order to ensure low dynamic EVM and an output signal that is not distorted by unwanted gain and phase variations, a PA is generally not used unless it is thermally stable. A common approach to avoid the problem of dynamic EVM is simply to wait until a PA is thermally stable before using it to amplify the signal. A second common approach is to speed up the gain and phase response of the PA by applying an external resistor and speed-up capacitor to provide more forward current earlier to the PA. Although the speed-up capacitor can improve the time response of the PA, as a passive mechanism it cannot provide the additional forward current until the RF input signal itself arrives. Consequently, the beginning of the RF signal data will suffer from some amount of dynamic EVM and the additional current may not be sufficient to bring the PA into a thermally stable state at a desired rate.
In a multistage PA, each amplification stage ordinarily has a different power range and hence different power and thermal characteristics. Typically the stages of a multistage PA are arranged from the smallest stage, having the lowest power level to the largest stage, having the highest power level, as the RF signal is amplified. According to the known solutions for dynamic EVM avoidance, external passive networks are used to either slow down the changing gain response of the first stage or speed up the change in the gain of the second and/or third stages. Another common approach is simply to wait until all of the stage amplifiers including the largest stage amplifier are thermally stable before using the multistage PA to amplify the RF signal.
The external passive networks according to the known solutions possess various packaging and performance compromises. External RC circuits applied to the first and/or second stage can result in reduced output power due to headroom issues of those RC circuits, and result in the addition of further components to the packaging. Use of the speed-up capacitor also requires additional package pins to connect the capacitor across the bias reference current internal port. Any solution utilizing fixed external passive networks requires extensive fine tuning and optimization in the prototype phase. Waiting for all of the stages to become thermally stable, although avoiding dynamic EVM, introduces undesirable delay.